1. Field of the Invention
The present invention relates to driving circuits and driving methods for driving a capacitive load, for example a driving circuit for displaying an image by applying a voltage to a capacitive load, such as an active-matrix liquid crystal panel, as well as to display devices provided with such a driving circuit.
2. Description of the Related Art
Liquid crystal display devices display images by applying a voltage corresponding to an input video signal to each video signal line provided in a liquid crystal panel. That is to say, to display images with the liquid crystal display device, a capacitive load including, for example, the pixel capacitance and the wiring capacitance of the liquid crystal panel is driven by a driving circuit. Such liquid crystal display devices, for example thin film transistor (TFT) based active-matrix liquid crystal panels (in the following also referred to as “TFT-LCD devices”), have the following configuration.
The liquid crystal panel of a TFT-LCD device (referred to as “TFT-LCD panel” below) includes a pair of substrates opposing each other (referred to as “first and second substrate” below). These substrates are fastened at a certain distance (typically several μm) from one another, and a liquid crystal material is filled between the substrates, forming a liquid crystal layer. At least one of these substrates is transparent, and when performing transmissive display, both substrates are transparent. TFT-LCDs are provided with a plurality of scanning signal lines arranged in parallel on the first substrate and a plurality of video signal lines intersecting perpendicularly with the scanning signal lines. In correspondence with each intersection of the scanning lines and the video signal lines, a pixel electrode and a pixel TFT serving as a switching element for electrically connecting the pixel electrode to the video signal line passing through that intersection are provided. The gate terminal of this pixel TFT is connected to the scanning signal line passing through this intersection, the source terminal is connected to the video signal line passing through this intersection, and the drain terminal is connected to the pixel electrode.
A common electrode serving as the opposing electrode for the entire screen is disposed on the second substrate opposing the first substrate. A common electrode driving circuit applies a suitable potential to this common electrode. Consequently, a voltage corresponding to the potential difference between the pixel electrode and the common electrode is applied to the liquid crystal layer. The optical transmittance of the liquid crystal layer is controlled by this applied voltage, so that it is possible to perform the desired pixel display by application of a suitable voltage from the video signal line.
Ordinary liquid crystal display devices, however, are driven by AC driving in order to suppress deterioration of the liquid crystal and sustain the display quality. Examples of AC driving schemes are frame inversion driving, 1H inversion driving, source inversion driving, and dot inversion driving. In frame inversion driving, the polarity of the voltage applied to the liquid crystal is inverted at each frame period of the video signal representing the image to be displayed. In 1H inversion driving, the polarity of the voltage applied to the liquid crystal is inverted at each horizontal scanning period (and at each scanning signal line) of the video signal, and the polarity is also inverted at each frame period. In source inversion driving, the polarity of the voltage applied to the liquid crystal is inverted at each vertical line of the image to be displayed, that is, at each video signal line of the liquid crystal panel, and the polarity is also inverted at each frame period. In dot inversion driving, the polarity of the voltage applied to the liquid crystal is inverted at each scanning signal line and at each video signal line, and the polarity is also inverted at each frame.
For example, in 1H inversion driving, the polarity of the applied voltage signal is inverted between positive and negative at each frame period, and the polarity is also inverted at each horizontal scanning period, as shown in FIG. 14A. Usually, in order to perform such a polarity inversion, the video signal lines are AC driven by the video signal line driving circuit (also referred to as “source driver”), and the common electrode is AC driven by the common electrode driving circuit, as shown in FIG. 14B. If also the common electrode is AC driven in this manner, then the amplitude of the pulse wave voltage outputted from the video signal line driving circuit is relatively small, for example 5 V. On the other hand, if the potential Vcom of the common electrode is fixed (i.e. DC driven), and 1H inversion driving or dot inversion driving or the like is performed, then the amplitude of the pulse wave voltage (video signal line potential Vs) that is outputted from the video signal line driving circuit is for example 10 V, as shown in FIG. 14C, and is about twice greater than when AC driving the common electrode. As a result, the power consumption of the video signal line driving circuit becomes large.
On the other hand, the following two methods are conceivable as methods to decrease the power consumption in the above-described liquid crystal display device: A first method is the method of performing precharging every time the polarity of the voltage applied to the liquid crystal is switched, and employs a circuit configuration as shown for example in FIG. 15 for each output of the video signal line driving circuit (see for example JP H07-134573A, and the corresponding U.S. Pat. No. 5,929,847 (the content of this U.S. patent is incorporated herein by reference)). With this circuit configuration, a video signal line driving circuit outputting a driving signal Sj to be applied to the video signal lines is provided, for each output terminal TSj, with a positive side switch SWP and a negative side switch SWN, which are substantially reciprocally turned on and off in order to invert the polarity of the voltage applied to the video signal line. The positive side switch SWP is controlled by a positive voltage application control signal φp as shown in FIG. 16A. When the positive voltage application control signal φp is at high level (H level), then the positive side switch SWP is turned on, and when the positive voltage application control signal φp is at low level (L level), then the positive side switch SWP is turned off. The negative side switch SWN is controlled by a negative voltage application control signal φn as shown in FIG. 16B. When the negative voltage application control signal φn is at H level, then the positive side switch SWN is turned on, and when the negative voltage application control signal φn is at L level, then the positive side switch SWN is turned off. Thus, the positive side and negative side switches SWP and SWN switch alternately between a period in which a positive voltage is applied to the video signal line so that a positive voltage is held by the pixel capacitance formed by the pixel electrode and the common electrode (referred to as “P period” below), and a period in which a negative voltage is applied to the video signal line so that a negative voltage is held by the pixel capacitance (referred to as “N period” below), as shown in FIG. 16D. In addition, between the P period and the N period, there is a period during which both the positive side switch SWP and the negative side switch SWN are turned off (both φp and φn are at L level), and the output buffers 41p and 41n of the video signal line driving circuit are electrically disconnected from the video signal line (this period is referred to as “OFF period” below), as shown in FIGS. 16A and 16B.
In this first method, in addition to the positive side switch SWP and the negative side switch SWN, a power source referred to as “precharge power source” and a switch SWS are provided. One end of the switch SWS is connected to a suitable position on the signal line connecting the point where the positive side switch SWP is connected to the negative side switch SWN to the video signal line of the liquid crystal panel, and the other end of the switch SWS is connected to the precharge power source. This switch SWS, which is turned on when the precharge control signal Scs shown in FIG. 16C is at H level, and is turned off when the precharge control signal Scs is at L level, operates in synchronization with the positive side switch SWP and the negative side switch SWN. That is to say, based on a precharge control signal Scs, this switch SWS is turned on within the OFF period that is inserted between the P period and the N period, so that the video signal line is precharged with the precharge power source. If the voltage Vpr of the precharge power source is zero, which is precisely the mean voltage value between the positive voltage and the negative voltage outputted from the video signal line driving circuit, that is, if the other side of the switch SWS is connected to the common electrode of the liquid crystal panel, then the voltage with which the output buffers 41p and 41n of the video signal line driving circuit are to drive the video signal line becomes about half of the voltage in the case that this method is not employed, and the power consumption is reduced accordingly. That is to say, By turning the switch SWS on during the OFF period, for example the period of transition from the P period to the N period, the potential of the video signal line is precharged to an intermediate potential, and after that, a negative voltage is applied from a video signal line driving circuit. Thus, the voltage with which the output buffer 41n of the video signal line driving circuit is to drive the video signal line becomes half the potential change amount when switching polarities as shown in FIG. 16D.
In the second method for reducing the power consumption in liquid crystal display devices, by forming a closed loop including the static capacitance of the liquid crystal (capacitance corresponding to the above-noted pixel capacitance) in the period corresponding to the above-described OFF period, the charge that has accumulated in this liquid crystal static capacitance is discharged, and thus a reduction of the power consumption is attained (see for example JP S53-124098A and the corresponding U.S. Pat. No. 4,196,432). FIGS. 17A and 17B show a simplified equivalent circuit illustrating this second method. In this second method, the liquid crystal static capacitance (LCD) Co is charged as shown in FIG. 17A in the period corresponding to the P period in the first method for example, whereas in the period corresponding to the OFF period in the first method, a closed loop including the liquid crystal static capacitance Co is formed as shown in FIG. 17B, and the charge that has accumulated in the liquid crystal static capacitance Co is discharged. Thus, the liquid crystal driving current is trimmed, and the power consumption of the liquid crystal display device is reduced.
As explained above, with these first and second conventional methods, a reduction of the power consumption can be attained by reducing the changes of the video signal line potential to be changed by the driving circuit. However, the effect of these methods is confined to a reduction of the power consumption based on making the change of the video signal line potential to be changed by the driving circuit half the potential change of the video signal line during the time when the polarity is inverted, and a further reduction of the power consumption could not be attained.